Scrutinizing the Alignment Limit in Two-Higgs-Doublet Models. Part 2: $m_H=125$ GeV (1511.03682v2)

Abstract: In the alignment limit of a multi-doublet Higgs sector, one of the Higgs mass eigenstates aligns in field space with the direction of the scalar field vacuum expectation values, and its couplings approach those of the Standard Model (SM) Higgs boson. We consider CP-conserving Two-Higgs-Doublet Models (2HDMs) of Type I and Type II near the alignment limit in which the heavier of the two CP-even Higgs bosons, $H$, is the SM-like state observed with a mass of 125 GeV, and the couplings of $H$ to gauge bosons approach those of the SM. We review the theoretical structure and analyze the phenomenological implications of this particular realization of the alignment limit, where decoupling of the extra states cannot occur given that the lighter CP-even state $h$ must, by definition, have a mass below 125 GeV. For the numerical analysis, we perform scans of the 2HDM parameter space employing the software packages 2HDMC and Lilith, taking into account all relevant pre-LHC constraints, constraints from the measurements of the 125 GeV Higgs signal at the LHC, as well as the most recent limits coming from searches for heavy Higgs-like states. Implications for Run 2 at the LHC, including expectations for observing the other scalar states, are also discussed.

• The paper establishes that a 125 GeV heavy Higgs can align with Standard Model couplings via thorough numerical scans of the 2HDM parameter space.
• It quantifies deviations in fermionic couplings, noting up to 20% discrepancies in Type II models that impact Higgs interactions.
• The research identifies promising LHC channels, such as gg → A → Zh, which offer practical search strategies for extra Higgs states.

Overview of the Alignment Limit in Two-Higgs-Doublet Models with $m_H=125$ GeV

The paper presented in this paper provides an in-depth analysis of the alignment limit in Two-Higgs-Doublet Models (2HDMs) with a focus on scenarios where the heavier CP-even Higgs boson ($H$) is aligned with the scalar vacuum expectation values and has a mass corresponding to the discovered 125 GeV Higgs boson. This aligns the couplings of $H$ with those predicted by the Standard Model (SM) for the Higgs, while introducing unique phenomenological predictions due to the presence of additional Higgs states in the spectrum.

The authors investigate 2HDMs of Type I and Type II, models distinguished by their specific Yukawa coupling schemes that prevent flavor-changing neutral currents at tree level. The paper emphasizes scenarios where alignment without decoupling occurs, meaning the additional scalar states are relatively light and accessible at current collider energies.

Key Results and Numerical Analysis

The research employs comprehensive numerical scans of the 2HDM parameter space, constrained by theoretical requirements such as vacuum stability and perturbativity, as well as empirical constraints from Higgs signal strength measurements and direct searches for additional states at the Large Hadron Collider (LHC).

1. Parameter Space and Mass Correlations: The paper delineates how different masses for the CP-odd Higgs ($A$) and charged Higgs bosons ($H^\pm$) can coexist with a 125 GeV $H$. It notably identifies that these masses remain constrained below approximately 630 GeV.
2. Deviations in Higgs Couplings: While the SM-like $H$ needs to have couplings to $W$ and $Z$ bosons close to unity due to alignment, deviations up to 20% are possible in the couplings to fermions, particularly in Type II models where constraints on the down-type quark Yukawa coupling are less stringent.
3. Implications for LHC Measurements: Potentially observable effects in the signal strengths of the 125 GeV Higgs boson, including deviations in loop-induced processes like $H \to \gamma \gamma$, are explored. The manuscript finds foundations for signals in rare decay processes and vector boson associated production, offering enriched contexts for LHC Run 2 tests.
4. Discovery Channels for Extra Higgs States: Significant attention is directed toward the phenomenology of the lighter CP-even $h$ and the CP-odd $A$. The process $gg \to A \to Zh$ is particularly promising, providing unique signatures that could validate the model specifics.

Theoretical and Practical Implications

The alignment presented ensures robust interactions with electroweak gauge bosons, akin to the SM Higgs, while feasible discrepancies in fermionic interactions highlight potential signals beyond the SM. These revelations contribute significantly to particle physics, proposing methods to reconcile the 125 GeV Higgs in a broader scalar sector framework.

In conclusion, this paper elucidates fundamental properties of non-minimal Higgs sectors under current experimental constraints. It establishes predictive correlations between various coupling constants and decay modes—offering not just a theoretical framework, but real collider search strategies to possibly detect additional Higgs bosons, thereby enriching and potentially redefining our understanding of electroweak symmetry breaking. Future experimental endeavors will reveal more, allowing further refinement or refutation of these model predictions, especially through precision measurements and dedicated searches at the LHC.